We have demonstrated that a collagen matrix may be required for the growth of some connective tissue cells but not their tumorigenic counterparts. A relative change in the rate of collagen synthesis in tumorigenic versus normal cells was indicated by a dose dependent response to cis-hydroxyproline. A new in vivo method has been designed to measure the invasivity of metastatic cell lines and the effect of chemotherapy by monitoring the elimination of injected labelled cells from lungs. The chick allantoic membrane was used as a rapid bioassay to differentiate between invasive and non-invasive cell lines. A study of cell lines derived from explants of adult connective tissue cells confirmed that some phenotypic alteration of cells can be correlated with their tumorigenic potential in vivo. Various cell types induced by subcutaneously implanted nylon wool have been identified at definite intervals up to 30 days. During the fourth week, the presence of endothelial cells has been demonstrated. Their induction coincided with increasing levels of proteases.